arXiv subjects

A. Serafin

Publications and source records attributed to A. Serafin.

13 recordsLinked to original sources

arXiv2023

First Demonstration of the HeRALD Superfluid Helium Detector Concept

The SPICE/HeRALD collaboration is performing R&D to enable studies of sub-GeV dark matter models using a variety of target materials. Here we report our recent progress on instrumenting a superfluid $^4$He target mass with a transition-edge sensor based calorimeter to detect both atomic signals (scintillation) and $^4$He quasiparticle (phonon and roton) excitations. The sensitivity of HeRALD to the critical "quantum evaporation" signal from $^4$He quasiparticles requires us to block the superfluid film flow to the calorimeter. We have developed a heat-free film-blocking method employing an unoxidized Cs film, which we implemented in a prototype "HeRALD v0.1" detector of ~10 g target mass. This article reports initial studies of the atomic and quasiparticle signal channels. A key result of this work is the measurement of the quantum evaporation channel's gain of 0.15 $\pm$ 0.01, which will enable $^4$He-based dark matter experiments in the near term. With this gain the HeRALD detector reported here has an energy threshold of 145 eV at 5 sigma, which would be sensitive to dark matter masses down to 220 MeV/c$^2$.

physics.ins-det↗

arXiv2022

The Thermal Conductance of Sapphire Ball Based Detector Clamps

In order to provide secure clamping with a low thermal conductance, some low temperature detectors are held with point contact sapphire ball clamps. While this method is increasingly common, the thermal conductance across this interface has not been well studied. We present a direct measurement of the thermal conductance of such clamps between 200 and 600~mK, with a clamping force of approximately 2.7~$\pm0.3$~N/clamp. The thermal conductance of a single sapphire-on-copper clamp was found to be 660$^{+360}_{-210}$~$(T/K)^{3.1}$~[nW/K]. For a sapphire-on-silicon clamp the conductance was found to be 380$^{+190}_{-120}$~$(T/K)^{2.8}$~[nW/K]. The conductance measured is consistent with thermal boundary resistance.

physics.ins-det↗

arXiv2022

Scintillation yield from electronic and nuclear recoils in superfluid $^4$He

Superfluid $^4$He is a promising target material for direct detection of light ($<$ 1 GeV) dark matter. Possible signal channels available for readout in this medium include prompt photons, triplet excimers, and roton and phonon quasiparticles. The relative yield of these signals has implications for the sensitivity and discrimination power of a superfluid $^4$He dark matter detector. Using a 16~cm$^3$ volume of 1.75~K superfluid $^4$He read out by six immersed photomultiplier tubes, we measured the scintillation from electronic recoils ranging between 36.3 and 185 keV$_\mathrm{ee}$, yielding a mean signal size of $1.25^{+0.03}_{-0.03}$~phe/keV$_\mathrm{ee}$, and nuclear recoils from 53.2 to 1090 keV$_\mathrm{nr}$. We compare the results of our relative scintillation yield measurements to an existing semiempirical model based on helium-helium and electron-helium interaction cross sections. We also study the behavior of delayed scintillation components as a function of recoil type and energy, a further avenue for signal discrimination in superfluid $^4$He.

physics.ins-det↗

arXiv2022

A backing detector for order-keV neutrons

We have designed and tested a large-area (0.15~m$^2$) neutron detector based on neutron capture on \ce{^{6}Li}. The neutron detector design has been optimized for the purpose of tagging the scattering angle of keV-scale neutrons. These neutron detectors would be employed to calibrate the low-energy ($<$100 eV) nuclear recoil in detectors for dark matter and coherent elastic neutrino nucleus scattering (CE$ν$NS). We describe the design, construction, and characterization of a prototype. The prototype is designed to have a tagging efficiency of $\sim$25\% at the relevant $\mathcal{O}$(keV) neutron energies, and with a mean capture time of $\sim$17$~μ$s. The prototype was characterized using a \ce{^{252}Cf} neutron source and agreement with the simulation was observed within a few percent level.

physics.ins-det↗

arXiv2019

Nuclear Recoil Scintillation Linearity of a High Pressure $^4$He Gas Detector

We investigate scintillation linearity of a commercial high pressure $^4$He gas detector using monoenergetic 2.8 MeV neutrons from a deuterium-deuterium fusion neutron generator. The scintillation response of the detector was measured for a range of recoil energies between 83 keV and 626 keV by tagging neutrons scattering into fixed angles with a far-side organic scintillator detector. Detailed Monte Carlo simulations were compared to experimental data to determine the linearity of the detector response by comparing the scaling of the energy deposits in the simulations to the detector output. In this analysis, a linear scintillation response corresponds to a consistent value for the scaling factor between simulated energy deposits and experimental data for several different scattering angles. We demonstrate that the detector can be used to detect fast neutron interactions down to 83 keV recoil energies and can be used to characterize low-energy neutron sources, one of its potential applications.

physics.ins-det↗

arXiv2015

Evidence for a New Intermediate Phase in a Strongly Correlated 2D System near Wigner Crystallization

How the two dimensional (2D) quantum Wigner crystal (WC) transforms into the metallic liquid phase remains to be an outstanding problem in physics. In theories considering the 2D WC to liquid transition in the clean limit, it was suggested that a number of intermediate phases might exist. We have studied the transformation between the metallic fluid phase and the low magnetic field reentrant insulating phase (RIP) which was interpreted as due to WC formation [Qiu et al, PRL 108, 106404 (2012)], in a strongly correlated 2D hole system with large interaction parameter $r_s$ ($\sim~$20-30) and high mobility. Instead of a sharp transition, we found that increasing density (or lowering $r_s$) drives the RIP into a state where the incipient RIP coexists with Fermi liquid. This apparent mixture phase intermediate between Fermi liquid and WC also exhibits a non-trivial temperature dependent resistivity behavior which may be qualitatively understood by the reversed melting of WC in the mixture, in analogy to the Pomeranchuk effect in the solid-liquid mixture of Helium-3.

cond-mat.str-el↗

arXiv2014

Competing quantum Hall phases in the second Landau level in low density limit

We present in this Letter the results from two high quality, low density GaAs quantum wells. In sample A of electron density n=5.0x10^10 cm^-2, anisotropic electronic transport behavior was observed at ν=7/2 in the second Landau level. We believe that the anisotropy is due to the large Landau level mixing effect in this sample. In sample B of density 4.1x10^10 cm^-2, strong 8/3, 5/2, and 7/3 fractional quantum Hall states were observed. Furthermore, our energy gap data suggest that, similar to the 8/3 state, the 5/2 state may also be spin unpolarized in the low density limit. The results from both samples show that the strong electron-electron interactions and a large Landau level mixing effect play an import role in the competing ground states in the second landau level.

cond-mat.mes-hall↗

arXiv2010

Anisotropic fluctuations and quasiparticle excitations in FeSe_0.5Te_0.5

We present data for the temperature dependence of the magnetic penetration depth lambda(T), heat capacity C(T), resistivity R(T) and magnetic torque ?tau for highly homogeneous single crystal samples of Fe1:0Se0:44(4)Te0:56(4). lambda(T) was measured down to 200mK in zero field. We find lambda(T) follows a power law lambda~T^n with n = 2.2 +/- 0.1. This is similar to some 122 iron-arsenides and likely results from a sign-changing pairing state combined with strong scattering. Magnetic fields of up to B =55T or 14T were used for the ? tau(B) and C(T)/R(T) measurements respectively. The specific heat, resistivity and torque measurements were used to map out the (H,T) phase diagram in this material. All three measurements were conducted on exactly the same single crystal sample so that the different information revealed by these probes is clearly distinguished. Heat capacity data strongly resemble those found for the high Tc cuprates, where strong fluctuation effects wipe-out the phase transition at Hc2. Unusually, here we find the fluctuation effects appear to be strongly anisotropic.

cond-mat.supr-con↗

arXiv2010

Destruction of chain-superconductivity in YBa_2Cu_4O_8 in a weak magnetic field

We report measurements of the temperature dependent components of the magnetic penetration depth λ(T) in single crystal samples of YBa_2Cu_4O_8 using a radio frequency tunnel diode oscillator technique. We observe a downturn in λ(T) at low temperatures for currents flowing along the b and c axes but not along the a axis. The downturn in λ_b is suppressed by a small dc field of ~0.25 T. This and the zero field anisotropy of λ(T) likely result from proximity induced superconducting on the CuO chains, however we also discuss the possibility that a significant part of the anisotropy might originate from the CuO2 planes.

cond-mat.supr-con↗

arXiv2010

Evidence for superconducting gap nodes in the zone-centered hole bands of KFe2As2 from magnetic penetration-depth measurements

Among the iron-based pnictide superconductors the material KFe$_2$As$_2$ is unusual in that its Fermi surface does not consist of quasi-nested electron and hole pockets. Here we report measurements of the temperature dependent London penetration depth of very clean crystals of this compound with residual resistivity ratio $>1200$. We show that the superfluid density at low temperatures exhibits a strong linear-in-temperature dependence which implies that there are line nodes in the energy gap on the large zone-centered hole sheets. The results indicate that KFe$_2$As$_2$ is an unconventional superconductor with strong electron correlations.

cond-mat.supr-con↗

arXiv2010

Line nodes in the energy gap of high-temperature superconducting BaFe_2(As_{1-x}P_x)_2 from penetration depth and thermal conductivity measurements

We report magnetic penetration depth and thermal conductivity data for high-quality single crystals of BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ ($T_c=30$\,K) which provide strong evidence that this material has line nodes in its energy gap. This is distinctly different from the nodeless gap found for (Ba,K)Fe$_2$As$_2$ which has similar $T_c$ and phase diagram. Our results indicate that repulsive electronic interactions play an essential role for Fe-based high-$T_c$ superconductivity but that uniquely there are distinctly different pairing states, with and without nodes, which have comparable $T_c$.

cond-mat.supr-con↗

arXiv2008

Evidence for nodal superconductivity in LaFePO

In several iron-arsenide superconductors there is strong evidence for a fully gapped superconducting state consistent with either a conventional s-wave symmetry or an unusual $s_\pm$ state where there the gap changes sign between the electron and hole Fermi surface sheets. Here we report measurements of the penetration depth $λ(T)$ in very clean samples of the related iron-phosphide superconductor, LaFePO, at temperatures down to $\sim$ 100 mK. We find that $λ(T)$ varies almost perfectly linearly with $T$ strongly suggesting the presence of gap nodes in this compound. Taken together with other data, this suggests the gap function may not be generic to all pnictide superconductors.

cond-mat.supr-con↗

arXiv2008

Magnetic penetration depth of single crystal SmFeAsO_{1-x}F_y: a fully gapped superconducting state

We report measurements of the in-plane magnetic penetration depth $λ$ in single crystals of SmFeAsO$_{1-x}$F$_y$ ($x\simeq y \simeq 0.2$) with $T_c \simeq 45$ K. We find that at low temperature $λ$ has an exponential temperature dependence which suggests that the Fermi surface is fully gapped. The magnitude of the minimum energy gap, $Δ_1=1.1\pm 0.1 k_BT_c$ at T=0 K, is significantly smaller than the BCS weak-coupling value suggesting that the gap is either strongly anisotropic or varies significantly between the different Fermi surface sheets. Our data fits well a two gap model with an additional larger gap of magnitude $Δ_2 = 1.8\pm0.2 k_BT_c$ which is associated with $\sim 80$% of the total superfluid density.

cond-mat.supr-con↗